In the field of flexographic printing ink samples are obtained by drawing ink over a substrate using a hand ink proofer, for example, of the type manufactured by Harper Companies International of Charlotte, N.C. Ink is applied to the substrate by manually rolling the hand proofer across the substrate. Manual ink proofer tools are utilized for proofing ink colors in order to accurately predict the results to be obtained by running a selected ink specimen in a printing press. A computer microscope is then used to view the ink smear on the substrate. The computer then indicates to the technician various color components to be added to the ink in order to achieve the desired ink coloration.
In a flexographic printing operation, rubber plates are utilized for delivering the ink to the stock or paper to be printed. A flexographic ink technician is usually given an ink specimen which has been determined to be acceptable for use on a particular press, and a production run sample, to be used as the standard for color and density. One of the most difficult tasks facing a flexographic ink technician is proofing ink in a manner so that the color will duplicate the color of the production run sample from the flexographic printing press. It is well known among those skilled in the art that if three trained technicians pull an ink proof, using the same ink on the same hand proofer tool, three different color shades will result.
The shade of a color on a flexographic printing press is dependent on the thickness of the ink film applied to the substrate or stock. The ink film thickness is determined by the speed of the press, the pressure applied between the printing plate and paper (i.e., impression), and the pressure between the rollers on the printing unit. Similarly, the shade of a color on a flexographic hand proofer tool is also dependent on the thickness of the ink film applied to the substrate which is determined by the speed at which the technician pulls the hand proofer tool across the substrate, and the impression pressure the technician applies to the hand proofer tool while moving it across the substrate. Thus, the speed and impression are totally dependent on the manual skill of the flexographic ink technician, while the only variable not controlled by the technician is the pressure between the ink roller and transfer roller of the manual proofer tool.
U.S. Pat. No. 6,814,001 describes an ink proofer designed to overcome the problems associated with conventional manual proofer tools by generating consistent and reliable ink draws using a hand-held proofer tool retained in a movable mounting assembly. A variable pressure system is coupled to the mounting assembly to move the proofer tool into a contact position with a cylindrical drum. The transfer roller of the proofer tool then transfers ink to a substrate inserted between the drum and the transfer roller of the proofer tool when a drive motor for the drum is engaged.
In prior art proofing tools the anilox roll and the impression roll are engaged to one another only by friction. A doctor blade removes excess ink from the anilox roller by scraping the anilox roller as it turns, and supports welled up ink to maintain a continuing supply of ink to replenish the anilox roller after ink has been transferred to the impression roller. Thus, there is a certain degree of doctor blade pressure on the anilox roller that tends to resist turning of the anilox roller.
In addition, nip pressure exists between the rollers. As the anilox roller and the impression roller meet the viscosity of the ink being transferred from the anilox roller to the impression roller tends to force the anilox roller and the impression roller apart. In conventional hand proofing tools, this force is countered by another force that arises because of the deflection of an adjustable spring in the handle of the tool.
It is desirable that the nip pressure between the anilox roll and the impression roll not be too high. It is known that the sheer force generated by a too high nip pressure between the anilox roll and the impression roll will change the sheer qualities of the ink and thus alter the appearance of the image on the sample that is pulled. It is also desirable to maintain the nip pressure on the proofing tool at a level very similar to the nip pressure on the printing press in order to obtain a similar appearing result between ink tested on the proofing tool and ink that is in production printed materials produced by the press.
In addition, the anilox roller and the impression roller are coupled only by friction. Printing ink may have significant viscosity. If nip pressure is maintained too low, the anilox roll will start skidding on the ink relative to the impression roller. In this circumstance, the impression roller will not be coated with ink properly and gaps will appear in the proof that is drawn. In a prior art proofing tool, doctor blade pressure and/or nip pressure can cause skidding between the anilox roll and the transfer roll.
Another issue arises because of slippage between the anilox and the impression roller is that transfer of ink from the anilox roller to the impression roller may vary, thus, causing variation in the proof produced.
Another issue arises with prior art hand proofing tools because it is desirable to separate the anilox roll from the impression roller when the proofer is not in use. If the anilox roll and the impression roll remain in contact with one another indentation of the impression roll or damage to the anilox roll will tend to occur thus causing an uneven transfer of ink and making the anilox roll impression roll assembly useless for providing a good proof. Prior art hand proofers generally include a release mechanism to release pressure between the anilox roll and the impression roll. However, this pressure release mechanism must be manually operated. If an operator forgets to operate the pressure release mechanism the rolls may be compromised.
Yet another issue that arises with prior art proofers is that if the proofer is set down on a surface the impression roller will make contact with that surface. This causes transfer of ink from impression rolls of the surface creating a mess that must be cleaned up and, in addition, may cause foreign material to be picked up on the surface of the impression roller which may then be turned and rotated into the anilox roller thus damaging the anilox roller or the impression roller or both.
Another shortcoming of many prior art hand proofers is that, when in use, the anilox and transfer roll are in a non-vertical orientation relative to one another. A printing press is arranged so that the anilox and impression roll are in a vertical position during use, thus, gravity affects the transfer of ink between the anilox and the transfer roll. In making a proofer that gives the most reliable possible proofs it would be desirable to duplicate the relationship between the anilox and the transfer roll that is seen in printing presses.